Heretofore, high-strength iron-based alloys have been used as power generation turbine components and aircraft body components.
In the power generation turbine components, high Cr steel is used for various kinds of the components. Among the turbine components, a low-pressure final-stage rotor blade of a steam turbine should be particularly strengthened. Thus, in this component, stainless steel containing approximately 12 weight % of Cr, 12-Cr steel, is used as an alloy having combined properties of strength, oxidation resistance, and corrosion resistance. Among them, furthermore, the blade having a longer blade length is advantageous to improve power generation efficiency. However, the length of the 12-Cr steel blade is limited up to about 1 meter because of its limited strength.
Also, there are known low alloy high tensile steels such as AISI4340 and 300M. These alloys are low-alloy steel capable of attaining a tensile strength in the order of 1800 MPa and an elongation of about 10%. In these alloys, however, the amount of Cr, which contributes to corrosion resistance and oxidation resistance, is as small as approximately 1%. Therefore, any of these alloys cannot be used as a steam turbine rotor blade. When applied to an aircraft application, these are also often subjected to surface treatment such as plating before use to prevent corrosion from salt or the like in the air.
On the other hand, as an alloy having combined properties of strength, corrosion resistance and oxidation resistance, there is a high strength stainless steel. Representative examples of the strengthening type martensitic steel known in the art include precipitation strengthening type martensitic steel such as PH13-8Mo (Patent Document 1 and Patent Document 2). The precipitation strengthening type martensitic steel, fine precipitates are dispersed and precipitated in a quenched martensite structure to obtain higher strength compared to quenching-tempering type 12-Cr steel. Furthermore, compared with the low-alloy steel, these are excellent in properties of corrosion resistance and oxidation resistance because of containing 10% or more of Cr that contributes to corrosion resistance.